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NF Measurements With a DSP Radio



Folks,

I am asking for noise figure advice again.  For those of you
who have knowledge in this area, please let me know if I am
headed in the right direction.

I did some testing this weekend to see if I could use the
DSP-10 DSP Transceiver along with a calibrated NoiseCom
noise generator as a "noise measuring instrument".

The measurements were made with the DSP-10 set as follows:
Mode=USB
Frequency=144.2 MHz (DSP-10 tune-up frequency)
Gain=100 (RF attenuator off)
AGC=off
ENR=6.6dB (noise generator output at 144 MHz)

Relative power in dB was read from the lower-right screen
display showing "relative average power of the entire
spectral display".

Since I had read that a correction sometimes needs to be made
for the noise generated by the measuring equipment I tried to
measure the DSP-10's noise figure first.

With the noise generator connected directly to the input of the
DSP-10, I read 9.45dB with the noise source off and 11.25dB
with it on.  This is a difference of 1.8 dB (a Y of 1.51 I think?).
Next I divided 6.6 by 1.51-1 arriving at 12.9dB as a noise figure
for the DSP-10.

Does a 12.9dB noise figure look right for the DSP-10?

Next I tried measuring an old home-built broadband MAR-6 amplifier.
To do this I simply inserted the amp between the generator and the DSP-10.

First, to check the gain of the amplifier, I turned on the generator
and took another reading.  The reading was 30.8dB.  I subtracted the
"ON" reading of the DSP-10 by itself (11.25dB) from the 30.8dB to arrive
at 19.6dB.  The published gain value for the MAR-6 at 100 MHz is 20dB
so I felt this was OK.

Next I read 25.79dB with the noise source off.  This is a difference
of 5.01dB, which I believe is a Y of 3.17.  Next I divided 6.6 by 3.17-1
to arrive at 3.0.  Since the published noise figure for the MAR-6 is 3.0dB
I thought that this was OK.

Adjusting for the noise figure of the measuring equipment gives a 2.9dB
noise figure for the MAR-6.

Is this measurement technique a valid one or am I barking up the wrong tree?

The MAR-6 results could certainly be coincidence.  It would not be the first
time I have been lulled into complacency by such results.

Any suggestions and/or critiques would be appreciated.

Thanks,
Bill, WA6QGR

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